The latest developments will be watched closely by current Sunshine Coast MP Mal Brough.

Mr Brough has admitted he asked Mr Slipper's staffer James Ashby to make copies of the Speaker's private diary.

Mr Brough confirmed his part in the plan to 60 Minutes in September last year, saying his actions were justified because Mr Slipper had committed a crime.

The Guardian reported the AFP is still investigating Mr Brough's involvement.

"The AFP can confirm that it received a referral regarding the alleged unauthorised disclosure of Mr Slipper's diaries on 8 September 2014 and is investigating this matter," an AFP spokesperson told The Guardian.

"This investigation remains ongoing."

Mr Slipper was under scrutiny at the time for misusing travel entitlements, following a major investigation by Sunshine Coast Daily journalist Bill Hoffman.

Mr Slipper was found guilty of misusing Cabcharge vouchers.

Former Peter Slipper staffer James Ashby on Channel Nine's 60 Minutes

In parallel, Mr Ashby had launched a sexual harrassment case against Mr Slipper, which was thrown out because the judge found it was "to pursue a political attack against Mr Slipper".

In February 2014, the Federal Court upheld an appeal by Mr Ashby against the earlier ruling.

As part of the court's decision, the judges included a view on Mr Brough's involvement:

"We are also of the opinion that there was no basis for the primary judge to conclude that Brough was part of any combination with anyone in respect to the commencement of these proceedings with the predominant purpose of damaging Slipper in the way alleged or at all."

Mr Slipper went on to win a legal fight to challenge his conviction in an Australian Capital Territory court.

The AFP investigation followed Mr Brough's admission on the Nine Network, which was referred to the authorities by Labor's Graham Perrett, a former Supreme Court solicitor.

During an interview with ABC's Radio National on Tuesday, Mr Brough said the allegations were "a misunderstanding" and he would not "go back and canvass all these issues".